The invention relates to a rotational molding process for molding articles, and more particularly to a mold and method for use in a rotational molding process in which fiber reinforced thermoset plastic material is used to fabricate the mold in lieu of conventional materials to facilitate design changes and prototyping with low cost tooling.
Rotational molding is a unique process, as compared to injection molding and blow molding. It allows the producer greater flexibility in end-product design, especially for hollow parts, beveled wall constructions and large sizes where conventional tooling would make end-product costs prohibitive. Products can be rotationally molded, ranging from small syringe bulbs to large storage tanks or shipping containers. Other typical products rotationally molded include bumpers, dolls, squeeze toys, basketballs, footballs, automotive armrests and headrests, and boat hulls.
In rotational molding, the product is formed inside a closed mold or cavity while the mold is rotating bi-axially in a heating chamber. Rotational molding is best suited for large, hollow products requiring stress-free strength, complicated curves, a good finish, a variety of colors, a controlled duration production run, and uniform wall thickness. There are basically four steps in rotational molding: loading, molding or curing, cooling, and unloading. In the loading stage, either liquid or powdered plastic is charged into a hollow mold. In the oven, heat penetrates the mold causing the plastic to adhere to the mold surface. Since the mold continues to rotate while heating is going on, the plastic will gradually become distributed evenly on the mold walls through gravitational force. As the cycle continues, the polymer melts completely, forming a homogeneous layer of molten plastic.
While rotational molding allows the designer much flexibility in the design of the end-product, the cost for tooling and molds is still relatively expensive and is too high to allow changes for new designs, short runs, custom work, or prototyping. Molds used in rotational molding have been constructed from one or more of several metals. Machined aluminum generally gives parts which are free of surface porosity or voids. Cast aluminum molds have been used for rapid heat transfer, and are probably the most widely used in rotational molding. Initial cavity cost may be relatively high since a model and/or pattern are required. Electroformed nickel molds are another type used in rotational molding. This type of mold is noted as more durable than cast aluminum. Sheet metal molds are also used in rotational molding, particularly for large parts or single cavities requiring inexpensive tools. Prototype molds are often fabricated in sheet metal for reasons of cost, though eventually production molds are usually made of cast aluminum. While sheet metal tooling is relatively cheap, it does not easily allow for curved and contoured molding. Because of the relative high cost of cast aluminum molds, the design of the molded part is usually not readily changed. Production must be run until the change in design and cost of a new mold can be justified. The marketing of rotationally molded parts becomes limited in terms of meeting rapidly changing tastes and conditions, and for short run custom designs. The conventional molds have also been expensive in terms of repair costs.
Typical metallic molds and processes for rotational molding are disclosed in U.S. Pat. Nos. 4,457,729, 4,247,279 (aluminum), 4,238,537, 3,843,285, 3,966,870 (nickel), 3,652,368, and 2,681,472 (steel).
Accordingly, an object of the invention is to provide an alternate, low cost mold and process for use in rotational molding which is simple and inexpensive to tool and fabricate.
Another object of the invention is to provide a mold and method for use in rotational molding which uses relatively low cost tooling to make feasible short run design changes or prototyping.
Another object of the invention is to fabricate molds for rotational molding using fiber reinforced thermoset plastics which are relatively simple and inexpensive to pattern and fabricate allowing the molded article to readily and quickly change in its design without prohibitive costing.
Yet another object of the invention is a mold and method for rotational molding which is fast, uses inexpensive tooling, and provides a wide variety of curved and contour shaping in the molded article.